Aging alters the individual's physiological and functional responses to external stimuli, including the responses to infectious agents and therapeutics including biological response modifiers. We therefore examined the effect of aging on immunity. As a model, we utilized the cytolytic T lymphocyte (CTL) response generated against alloantigens. In one series of experiments involving the analysis of the response to multiple alloantigens including MHC, we found that CTL generated by cells from either young or old mice utilized CD8-positive T effector cells and CD4-positive T helper cells. Age-related deficits in the CD4-positive T cell subset as well as deficits in the CD8-positive effector cells were responsible for the reduced CTL generated by T cells from aged mice. The deficit could not be attributed to the well-established reduction with age of IL-2 synthesis, nor an hypothesized increase in IL-10 (cytokine synthesis inhibitory factor) synthesis. As a continuation of our finding that perforin mRNA synthesis is reduced with age, we have observed a corresponding reduction in the number of cells producing perforin protein. This reduction was found in CD8- positive T cells, in lymph node as well as spleen cells, and was not fully evident until the end of the time required for CTL generation, suggesting that aging alters the vector along which CD8-positive T cells differentiate into fully mature CTL. In a series of collaborative experiments, we examined allogeneic CTL generated in response to antigens stimulating either CD4-positive or CD8- positive T helper and T effector cell function. We found that the age- related effect was most evident in the CD4-positive T cell-dependent system. These results were obtained both by measuring CTL generated in vitro and skin transplant rejection in vivo, supporting the relevance in vivo of the age-related decline in CTL activity, and of the age-related decline in the function of CD4-positive T cells.